Internal combustion engines can typically be characterized as corresponding to one of two types of engines. In spark-ignited internal combustion engines, a mixture of fuel and air is compressed without causing ignition or combustion of the air/fuel mixture based just on compression. A spark is then introduced into the air fuel mixture to start combustion at a desired timing.
The other typical type of engine is a compression ignition engine. In compression ignition, combustion air is compressed in the cylinder, resulting in an increase in the temperature and pressure of the air. Fuel is injected into the hot compressed combustion air and auto-ignition occurs. A fuel for a compression ignition engine can be characterized based on a cetane number, which is a measure of how quickly a fuel will ignite. Many conventional compression ignition engines use kerosene and/or diesel boiling range compositions as fuels. However, some compression ignition engines can use naphtha boiling range compositions as fuels. Examples of advanced combustion engines that can use naphtha boiling range fuel compositions include, but are not limited to, homogenous charge compression ignition (HCCI) engines and pre-mixed charge compression ignition (PCCI) engines.
Spark ignition engines can have improved operation when operated with a fuel that provides a sufficient ignition delay so that the start of combustion is substantially controlled by the introduction of a spark into the combustion chamber. Fuels that do not have a sufficient ignition delay for an engine can cause “knocking” in the engine, where at least part of the combustion in the engine is not dependent on the introduction of the spark into the combustion chamber.
Traditionally, fuels for spark ignition engines have been characterized based on use of octane ratings, which is a measure of the ability of a fuel to resist combustion based solely on compression. The octane rating is valuable information for a spark-ignited engine, as the octane rating indicates what type of engine timings may be suitable for use with a given fuel. A common method for characterizing the octane rating of a fuel is to use an average of the Research Octane Number (RON—ASTM D2699) and the Motor Octane Number (MON—ASTM D2700) for a composition. (RON+MON/2). This type of octane rating can be used to determine the likelihood of “knocking” behavior when operating a conventional spark ignition engine.
Another type of characterization of a fuel for a spark ignition engine is the sensitivity of the fuel, which is defined as (RON−MON). Some previous methods for selecting fuels with longer ignition delays at a given value of RON have involved selecting fuels with lower values of the sensitivity.